Light-emitting devices (LEDs) comprising conjugated polymers have become a topic of great interest since the first demonstration of an efficient polymer light-emitting diode. A large variety of polymers, copolymers, and their derivatives have been shown to exhibit electroluminescent properties. Single layer polymer LEDs are typically constructed by sandwiching a thin layer of luminescent conjugated polymer between two electrodes composed of an anode and a cathode, wherein at least one electrode is either transparent or semi-transparent.
Light-emitting layers in the form of a film of a conjugated polymer are used in electroluminescence devices. Examples of such devices are described in WO 90/13148 and EP-A 0 443 861. The configurations of these devices may consist of a simple single layer, bilayers, or blends used to enhance efficiency and tune the emission wavelength, or multilayers that may allow the device to be operated under an applied voltage.
Although good results have been obtained with the application of conjugated polymers in devices, the purity of the emission colour, in particular, is still unsatisfactory. Full colour displays require pure red, green, and blue emission. Obtaining monochromatic emission from conjugated polymers or small organic molecules is still difficult, since their emission spectra typically have a line width (full width at half maximum, or FWHM) of 50-200 nm, arising from inhomogeneous broadening and vibronic progression. Furthermore, in conjugated molecules, light is only generated from the singlet excitons, while the triplet excitons are lost in the non-radiative transitions. From spin statistics, since only 25% of the excitons in conjugated molecules have singlet character, the quantum efficiency of a conjugated polymer LED cannot exceed 25%.
In attempting to optimise monochromatic emission, a great deal of effort has been devoted to the application of rare earth complexes in organic LEDs. Rare earth compounds are excellent chromophores that exhibit intense fluorescence with a narrow spectral bandwidth (FWHM of 5 to 20 nm) and relatively long decay lifetime (10−2 to 10−6s). They are the most widely used materials in CRT displays and inorganic LEDs.
However, the organic LEDs, especially the single-layer LEDs, based on rare earth complexes show relatively low efficiency as well as poor performance. These phenomena may be attributed to the poor ability of organic LEDs to transport charge carriers and inefficiency in energy transfer. Additionally, many of the low molecular weight rare earth complexes undergo decomposition to some extent during film formation by vacuum deposition. Decomposition of the rare earth complexes can be avoided in spin-coated films containing the rare earth complexes dispersed in polymer matrices. A drawback of this technique, however, is that non-uniform blending or dispersion of the dopants may result in phase separation and ionic aggregation.
WO 02/43446, JP 2000327715, and U.S. Pat. No. 6,148,142 describe polymeric compounds, wherein rare earth complexes are either blended or doped with the polymers, or chelated to non-conjugated polymers. However, these polymers have several drawbacks such as low efficiency and broad emission bands.
Accordingly, there is a need in this field of technology of improved and more efficient polymers, for example, polymers having improved semi-conductive properties and/or mechanical flexibility.